Is your sample data similar to your actual data?
Because your PO Information table doesn't have a unique id of any kind. Based on the sample data, I don't see how you could create any kind of relationships because they would all be many-to-many.
That being said, if your actual data is structured in a usable way, you don't need to do any kind of join or merging.
Simply go to a sheet, add a datasource and select one of your tables.
Then click the sheet, select 'add a new source' and select your next table. Repeat until all three of your tables are in there.
Now, you can select 'Data' >> 'Edit Relationships' and then select how your two sub-tables relate to your primary table via which keys they relate on.
Now, when you drag a measure or dimension from your main table, and you want to include something from one of your other tables, that relationship will initiate. You'll notice these red little paper-clip looking things next to the fields you related on.
Hopefully that helps.